Experiments using mouse teratoma cells are designed to study events of cellular and X-chromosomal differentiation. Embryonal teratoma cells having multiple potentials for cellular differentiation will be subjected to various environmental influences designed to preferentially elicit specific differentiations. Selective conditions for cells able to express specific differentiations will be applied. Teratomas will be induced in a variety of inbred mouse strains to produce tumor lines having chromosomal and biochemical markers. For purposes of X-chromosomal studies, teratomas will be induced in mice having morphologically and genetically distinguishable X chromosomes. The expression of X-chromosomal genes will be studied in cultured female cells derived from teratoma embryonal cells and normal mouse cells with the aim of comparing the state of expression of the two X-s in cells having somatic or germinal origins and potentialities, and to determine the stability of expression of X-chromosomal genes in cultured cells. For these purposes, experiments using cell cloning and cell hybridization techniques are outlined. The derivation of biochemically deficient teratoma cells is described, as are experiments to determine the capacity of teratoma cells to participate in the formation of chimeric mice. The aim of these experiments is to create defined mutations that may be analyzed for their effect on the development of the whole organism.